As is well known, carbon dioxide and ammonia can be caused to react with each other to form ammonium carbamate. It is also known that ammonium carbamate can be converted into urea and water. Moreover, urea can be produced directly from carbon dioxide and ammonia by causing the reaction to take place at a suitable temperature and pressure and for a sufficient period of time to allow the initially formed ammonium carbamate to be converted to urea. Typically, temperatures greater than 150.degree. C. and pressures greater than 10MPa are used. This direct process is the basis for most commercial synthesis of urea at the present time. It is, however, also known that at any commercially practical suitable temperature and pressure, the percentage conversion, (which is the proportion of carbon dioxide fed to the process which is converted to urea, expressed as a percentage) is limited. It is further known that this limit is essentially due to an equilibrium being established as a result of the reverse reaction between water and urea. According to the most recent correlation in the literature (D. M. Gorlovskii and V. I. Kucheryavyi, Zhurnal Prikladnoi Khimii 53.11, 2548-2551 November 1980) the maximum possible conversion to urea at equilibrium is close to 86 percent, however the highest experimentally observed conversion to urea is close to 84 percent. Several of the most recent urea processes have been described by I. Mavrovic and A. R. Shirley in an article (Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition, Published by John Wiley & Sons New York (1983) Vo. 23, p. 548-575), which indicates that in the present state of art, the highest conversion of carbamate to urea in a single pass is achieved by the UTI Heat-Recycle Process, with a reported 72-74 percent conversion per pass. In U.S. Pat. No. 3,236,888 (Wentworth) the only example cited gives the conversion to urea based on carbon dioxide introduced to the reactor as about 76 percent per pass. E. Guccione, (Chemical Engineering, Sept. 26, 1966 p. 96-98), in discussing this same process claims that the higher temperatures (380.degree. to 450.degree. F.) permit high conversion of CO.sub.2 to urea of 80 to 85 percent.
It is also known from the literature (see Krase and Gaddy, Journal American Chemical Society 52, 3088-3093 (1930)) that attempts have been made to remove water produced by the above described process by using dehydrating agents, in either the gas or liquid phase, to increase the conversion of carbamate to urea.
In Ruf et al., (Swiss Chem 6 (1984) Nr. 9, 129-141 and Swiss Chem 8 (1986) Nr. 10a, 18-25) there is a theoretical description relating to simulation of a urea plant and enhancement of the yield based on a hypothetical new technology in which a semipermeable membrane selectively removes water from urea melts. It is stated in the latter paper that a reverse osmosis process is visualized in which the water is removed, across the membrane which forms the wall of the reactor, by means of the high pressure in the reactor and which overcomes the osmotic pressure gradient.
The paper states that no membrane is known to the authors which is capable of carrying out the process, but the future development of such is forseeable. It is further stated that the simulation, and computationally demonstrated benefits and improved methods of operation theoretically possible with the process, are not confined to the particular membrane process, but apply to any workable water removal process.